While, as will be better understood from the following description, the present invention was developed for use in cardiac defibrillators, it is to be understood that the invention may also find use in other environments where the transfer of high energy electric power is required.
One of the most common and life-threatening medical conditions is ventricular fibrillation. A human heart experiencing ventricular fibrillation is unable to pump the volume of blood required by the human body. Loss of blood flow quickly leads to serious brain damage. Death will invariably result unless a normal heart rhythm can be restored within a short period of time. Ventricular fibrillation can result from a heart attack, or be caused by accidental electric shock or severe stress of the type that can accompany surgery, drowning or the like.
The usual way of restoring a normal rhythm to a heart experiencing ventricular fibrillation is to apply a strong electric pulse to the heart using an external cardiac defibrillator. External cardiac defibrillators have been successfully used for many years in hospitals by doctors and nurses and in the field by emergency treatment personnel, i.e., paramedics.
Conventional external cardiac defibrillators first accumulate a high energy electric charge on a storage capacitor. When a switching mechanism is closed, the stored energy is transferred in the form of a large current pulse to a pair of electrodes positioned on the chest of a patient. The switching mechanism used in most contemporary defibrillators is a heavy duty electro-mechanical relay, in many instances a rotary relay. A discharge control signal causes the relay to complete an electrical circuit between the storage capacitor and a wave shaping circuit whose output is connected to the electrodes attached to the patient.
Although the relays previously used in cardiac defibrillators have performed satisfactorily, they have a variety of disadvantages. The major disadvantage is cost. More specifically, prior art rotary relays used in external cardiac defibrillators have had high part counts, making them expensive to manufacture. Not only are the parts expensive to manufacture, they require a large amount of assembly and adjustment time. Further, prior art cardiac defibrillator high energy transfer relays have been designed such that the entire relay must be replaced even if only a small part fails, making replacement expensive. The second disadvantage is the size and weight of prior art cardiac defibrillator high energy relays. Because most external cardiac defibrillators are intended to be portable, they should be small in size and weight. Size and weight are functions of the parts used to make cardiac defibrillators. Because prior art high energy transfer relays used in portable cardiac defibrillators have been large and heavy, previously developed portable cardiac defibrillators have been heavier than desired. The weight and size of the rotary relays previously used in external cardiac defibrillators have made such relays difficult to mount, particularly on circuit boards. One way to overcome the cost, size and weight disadvantages of the high energy relays presently used in cardiac defibrillators is to use linear solenoid relays. Unfortunately, in the past, linear solenoid relays have had a number of disadvantages that have made them unsuitable for use as high energy transfer relays in cardiac defibrillators.
When a relay is closed, a switching contact is moved into engagement with a stationary contact. In the past, the contacts of both rotary and linear solenoid relays have had little resiliency and because the switching contact is moving at a high rate of speed, the momentum of the switching contact causes the switching contact to bounce away from the stationary contact after initial engagement. The bounce energy is offset by the relay closure energy, which brings the contacts back into engagement, causing a second smaller bounce and the cycle to be repeated. The switching contact bounces transiently against the stationary contact for a period of time before settling down. Arcing across the contacts occurs as the contacts bounce. Arcing has three undesirable effects. First, arcing may distort the shape of the current pulse applied to patient. Second, arcing may cause contact pitting, contact burning, or may weld the contacts together. Third, arcing can cause electromagnetic interference (EMI), which can be detrimental to the signals used by nearby control circuits.
The major disadvantage weighing against the use of linear solenoid relays in external cardiac defibrillators is inadvertent discharge of the storage capacitor. Most external defibrillators are portable and used by rescue personnel, such as fire and ambulance personnel. During transportation, defibrillators are subject to numerous shocks or jars that could cause the contacts of a linear solenoid relay to close and, thus, the associated storage capacitor to discharge. The inadvertent production of a high energy current pulse can create a hazard for rescue personnel.
The present invention is directed to providing a linear solenoid high energy transfer relay that overcomes the foregoing and other disadvantages, making it ideally suited for use in external cardiac defibrillators.